Strands or cables made of plurality of high-grade interwined steel wires are frequently used in prestressing of concrete structures, on account of their elevated bearing capacity accompanied by ease of handling. The anchoring of the strands to the concrete is brought about either by means of round keys in an anchoring plate or by making use of the bond. To transfer the tension load from the strand or cable to the concrete by bonding, one requires a greater transfer length. Upon embedding of the undeformed strand or cable, the required feed length is frequently not available; in such instances, anchoring must be brought about by costly anchoring plates and round keys. Besides, a short transfer length provides also the advantage of savings of costly strand. Since, frequently, a large number of strands comes to be anchored in a concrete structure, it must be possible to perform quickly and simply the measures resulting in a shortening of the feed length. Any reduction of the feed length results necessarily in an increase of the cleavage forces in the concrete. The bonding aids used must not bring about any excessive increase of the cleavage forces since, otherwise, additional reinforcement steels are required to prevent a cracking of the concrete. Moreover, the mechanical properties of the strand must not be significantly impaired by the anchoring measures.
Various measures are known that are designed to achieve an improved bond of the strand; e.g. in the publication "Betonwerk and Fertigteil-Technik", No. 6/76, there is described a so-called "bulb-type anchoring". In that case, the strand is untwisted by fanning it out and an attachment is inserted as a result of which there is brought about a local thickening of the strand accompanied by elastic deformation of the wires. This is a labor-intensive procedure. In order to render the bulb-type anchoring effective, it is in addition necessary that, downstream of or extending behind the bulb, there should still be available a certain length of strand that has not been untwined. That projection of strand increases the anchor length.
The affixing of shafts, loops, and hooks customarily used for the anchoring of steel rods does not bring about a reliable and efficient anchoring of the strands in view of the fact that, in those cases, the core wires of the strand do not come in direct contact with the concrete. In the case of the prestressing method accompanied by subsequent bonding, there are known in the art loop-type anchors in which the strands, coming from a sleeve, are guided within the concrete to form a loop and are then reinserted into the sleeve. It is however difficult to produce such loops. Frequently, it is even necessary to reduce the bearing pressure of the strands on the concrete by means of inserted sheet-metal elements.
The German Disclosure OS No. 2,557,072 describes an anchor in which the extremity of the strand is being upset in such a way as to produce a twin-cone-shaped enlargement made out of fanned-out stranded wires. This anchor resembles the bulb-type anchoring described above. The required projection of the strand downstream of the twin-cone-shaped enlargement is reduced by the applying of counter-bending onto the fanned-out stranded wires. A disadvantage of this anchoring is the comparatively substantial length of the "bellying" and of the projection of the strand at a given diameter of the enlargement. There exists also the danger that at the beginning of the bulge--where the entire or substantially the entire tension force is still present--the plastic deformation of the stranded wires is so intense that tensile and dynamic strengths are reduced excessively.